1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic process or electrostatic recording process such as a copying apparatus or a printer.
2. Related Background Art
In a prior image forming apparatus such as a copying machine, a printer or a facsimile, there is known a color image forming apparatus employing, for the purpose of reducing the dimension of the entire apparatus, a method of transferring a toner image, formed for example by an electrophotographic process on an image bearing member, onto an intermediate transfer member and transferring the toner image, transferred to the intermediate transfer member, onto a transfer material.
FIG. 9 is a view showing a configuration of an image forming unit of a color image forming apparatus provided with an intermediate transfer member which is a background technology of the present invention. As shown in FIG. 9, an intermediate transfer belt unit 31 is provided with an intermediate transfer belt 30 constituting an intermediate transfer member, and is detachably mounted in a main body of an unrepresented image forming apparatus. The intermediate transfer belt unit 31 can be detached by opening a unit replacement door provided in the main body of the unrepresented image forming apparatus, and by extracting the unit in a direction indicated by an arrow a.
The intermediate transfer belt 30 is supported by a drive roller 100, a tension roller 105 and a secondary transfer backup roller 108 provided in a belt frame 110, and is rotated by the drive roller 100 in a direction indicated by an arrow. The intermediate transfer belt 30 is given a tension by being biased, across a tension roller 105, by a compression spring 123 which presses a shaft 105a of the tension roller 105.
On the other hand, between the tension roller 105 and the drive roller 100 and along the intermediate transfer belt 30, there are provided plural image bearing members for forming toner images, for example four photosensitive drums 26 for yellow (Y), magenta (M), cyan (C) and black (Bk). In FIG. 9, there are only illustrated photosensitive drums 26Y, 26Bk for the first and fourth colors.
Also, inside the intermediate transfer belt 30, there are provided plural transfer rollers 102 for executing primary transfer of the toner images of respective colors on the photosensitive drums 26, in opposed relationship to the photosensitive drums 26. In FIG. 9, there are only illustrated transfer rollers 102Y, 102Bk for the first and fourth colors.
In the color image forming apparatus of such configuration, the toner images of four colors respectively formed on the four photosensitive drums 26 are transferred, by a transfer voltage applied to the transfer rollers 102, in superposition onto the intermediate transfer belt 30. Then, the toner images of four colors, transferred in superposition onto the intermediate transfer belt 30 are collectively secondarily transferred by a secondary transfer roller 127, opposed to the secondary transfer backup roller 108 across the intermediate transfer belt 30.
On the other hand, a transfer residual toner, remaining on the surface of the intermediate transfer belt 30 after the secondary transfer, is scraped off by a blade 120 mounted on a cleaning container 124, and the scraped toner is carried by a carrier 121 to an unrepresented user toner box. The blade 120 is biased toward the tension roller 105 thereby securing a contact pressure with the intermediate transfer belt 30.
FIG. 10 is a view showing a configuration around a secondary transfer area in a state where an image forming operation is not carried out. As shown in FIG. 10, in a state where an image forming operation is not carried out, the secondary transfer roller 127 is separated from the intermediate transfer belt 30.
The secondary transfer roller 127 is provided in a secondary transfer unit 133, which provided with a pressurizing mechanism for the secondary transfer roller 127 at both ends thereof. The pressurizing mechanism is constituted of a secondary transfer roller bearing 148 constituting a member to be pressurized, a roller pressing arm 147 constituting a pressurizing member, a roller pressing spring 142, and a contact-separation cam 160.
The contact-separation cam 160 is rotated by half turns by an unrepresented cam shaft rotating mechanism, in response to a contact-separation control signal. In every half turn of the contact-separation cam 160, the roller pressing arm 147 biased by the roller pressing spring 142 executes a vertical rocking motion. As a result, the secondary transfer roller bearing 148 executes a vertical movement and the secondary transfer roller 128 repeats a contact-separation operation with the intermediate transfer belt 30.
FIG. 11 is a view showing a configuration around a secondary transfer area in a state where an image forming operation is carried out. An image formation is conducted in a state where the contact-separation cam 160 is half turned from a state where the image forming operation is not carried out as shown in FIG. 10, whereby the secondary transfer roller 127 is contacted with the intermediate transfer belt 30.
A transfer material S, conveyed by an unrepresented transfer material feeding unit, is corrected from a skewed movement by impinging on a pair of registration rollers 24 which are in a non-rotated state. Then, in synchronization with the toner images on the intermediate transfer belt 30, rotation of the paired registration rollers 24 is initiated to convey the transfer material S to a secondary transfer area. The transfer material S, conveyed to the secondary transfer area, is pinched between the intermediate transfer belt 30 and the secondary transfer roller 127 rotated in contact with the intermediate transfer belt 30, and a transfer bias is applied to the secondary transfer roller 127 whereby the toner images on the intermediate transfer belt 30 are collectively secondarily transferred onto the transfer material S.
The application of the transfer bias to the secondary transfer roller 127 is achieved by applying a bias to one (at the deeper side of the apparatus) of two bearings 148 for the secondary transfer roller, positioned on both ends of the secondary transfer roller 127. More specifically, by an unrepresented high voltage supply unit, a transfer bias is applied from the main body of the image forming apparatus to a transfer roller contact plate 117 to supply a voltage to the secondary transfer roller 127 through a convex portion 148a of the secondary transfer roller bearing 148 and the secondary transfer roller bearing 148 whereby the secondary transfer is conducted. Therefore the secondary transfer roller bearing 148 is formed as bearing of a conductive resin, and the convex portion 148a is also formed with a conductive resin. Also in a pressing portion of the roller pressing arm 147 for pressurizing the secondary transfer roller bearing 148, a transfer roller contact plate 117 for bias supply, having an KN plating (chemical nickel plating), is supported by a contact holder 116 under an insulating condition.
The transfer material S, after the secondary transfer, is conveyed through a post-transfer conveying path 34 to an unrepresented fixing device and is subjected to a pressure and heating whereby the toner images are fixed to the transfer material S.
A registration unit 32 supporting the paired registration rollers 24 is provided with a pre-registration sensor flag 41 and a photointerruptor 40a in order to detect presence/absence of the transfer material S and a timing of arrival. Also in the post-transfer conveying path 34, a post-transfer sensor flag 43 and a photointerruptor 40c are similarly provided in order to detect presence/absence of the transfer material S and a timing of arrival.
A secondary transfer unit 133 executes a rocking motion in a counterclockwise direction in the drawing, by a weight thereof, about a rocking shaft 146, whereby the secondary transfer roller 127 is rendered movable to a retracted position (cf. FIG. 12) sufficiently separated from the intermediate transfer belt 30.
In case of processing a jammed transfer material S remaining in the apparatus by a conveying failure, the user at first executes an operation of moving the secondary transfer unit 133 to the retracted position and executes a processing of a jammed sheet in a space obtained in the secondary transfer area. However, in case the processing of a jammed sheet is difficult to conduct, it is also possible to conduct the jam processing after moving the intermediate transfer belt unit 31 in a direction a shown in FIG. 9 and detaching it to the exterior of the image forming apparatus. After the intermediate transfer belt unit 31 is returned to the main body of the image forming apparatus subsequent to the jam processing, the secondary transfer unit 133 is moved to a position capable of image formation.
Also at a replacement of the secondary transfer roller 127 which is a consumable component, the user moves the secondary transfer unit 133 to the retracted position, then pulls out the secondary transfer roller unit 161, which supports the secondary transfer roller 127 and the secondary transfer roller bearing 148, to a front side in the axial direction (cf. FIG. 13), and executes the replacement of the secondary transfer roller 127.
However, in the above-described apparatus of the background technology, there may result a drawback that toner dropping from the secondary transfer area, fine powder generated from the transfer material S and toner scattered in the apparatus are deposited on the transfer roller contact plate 117 to result in an unstable electrical conduction between the secondary transfer roller bearing 148 and the transfer roller contact plate 117, thus generating a failure in the electrical contact.
Also, since a sufficient transfer pressure is required for securing a transfer property even on a transfer material with a coarse surface, the pressurization of the secondary transfer roller 127 is made with a high leverage ratio utilizing the roller pressing arm 147. As a result, in a contact state of the secondary transfer arm 127 as shown in FIGS. 10 and 11, the moving direction of the secondary transfer roller bearing 149 does not become parallel to the pressurizing direction of the roller pressing arm 147 on the secondary transfer roller bearing 148. Therefore a rotary moment is generated on the secondary transfer roller bearing 148 to cause a deformation of the transfer roller unit 161, thereby inhibiting the contact-separation operation of the secondary transfer roller 127.
Furthermore, because of an influence in a play in the fitting of the secondary transfer roller bearing 148 in the secondary transfer roller unit 161, a position (action point) pressing the secondary transfer roller bearing 149 on the transfer roller contact plate 117 fluctuates on each contact-separation operation of the secondary transfer roller 127, thereby aberrating, though slightly, the alignment of the secondary transfer roller 127 with respect to the secondary transfer backup roller 108.